Many zero turn lawn mower manuals say to change the oil every 50 to 100 engine hours, however if you are using the Craftsman zero turn mower for residential use you will probably not reach that time for a couple of seasons.
Change the oil at the start of every season using 30W oil.
The model and serial numbers on your Craftsman zero-turn mower can be found on the rear of the frame by the engine.
NOTE: if you need a part for the small engine in your Craftsman zero-turn mower, there will be a separate model/serial number for the engine. Here are the small engines that are available in your Craftsman mower. Visit the page of the engine you have to find out where the model/serial number is located for your small engine model: